Intensive introduction of advanced composite materials in primary structures has became a fundamental approach for structural optimisation (based upon weight saving and improve performances), one of the priority goals in the design and manufacturing of new generation of commercial aircrafts. The implementation of an effective structural health monitoring (SHM) system, capable of predict failure of load paths in a structure designed following damage tolerance criteria would allow to optimise its design, and consequently reducing its weight. Fibre optic Bragg grating sensors (FBGs) have ability to measure mechanical strain with multiple additional advantages over electrical gauges, being considered, at the present time, the most qualified candidates to configure the new generation in-flight load monitoring systems. However, strain and load monitoring only allows predicting fatigue life until fracture for a certain probability of occurrence, and the distributed character of FBGs difficults the detection of local damage events, which only have effects on the near strain and stress fields, thus these devices has been then only anecdotally considered as damage sensors.
However, there are cases in which FBGs can be effectively used as damage sensors: monolithic structures composed by a skin with attached stiffeners or stringers: the failure of one these elements (by debonding, delamination of the skin or the flange of the reinforcing element, or even its breakage) causes a complete load re-distribution than can be easily detected by a simple strain sensing network, as the points required to be covered is limited. Moreover, the different stiffness of skin and stringer is associated to the storing of residual stresses/strains, which release when the part fails. This effect would allow the detection of the failure event in a known load condition (a reference condition), analysing the release of the residual stresses/strains by measuring its effect over the spectrum of a nearby FBG. This technique may be extended to composite and metallic structures with composite repair patches.
The ideas summarised above allows to establish the basis for a complete FBG based structural health monitoring system, based in the complete analysis of the spectral behaviour of the FBGs integrated in a structure, based in the following principles:
1. Detection of uniform longitudinal strain/stress release before and after damage occurrence, which promotes spectral shifts in nearby FBGs.
2. Detection of non-uniform longitudinal/transverse strain/stress release, which promote spectral distortions in nearby FBGs.
3. Detection of transverse strain/stress release, which promotes birefringence effects in nearby FBGs, with a subsequent influence in the spectral peak splitting.
The measurement of uniform strain is well known and straightforward, so most of efforts have to be oriented to characterise FBGs spectral distortions promoted by strains and stresses fields perturbations (discrimination of phenomena, repetitivity of the correspondence event severity/signal level, detection of false alarms, etc.).
A first and simple approach to this method requires only from the record and analysis of the evolution of the spectra of the FBGs at equivalent loading conditions (or at a known reference load condition), after service. Depending on the demodulation technique used, different type and amount of data are acquired, different methods of analysis have to be implemented, and different quality of information is obtained. For instance: a full spectrum acquisition facilitates the discrimination of phenomena, whereas peak detection based systems only allows detecting strong amplitude/wavelength variations.